1. Field of the Invention
The present invention relates to a four-cycle engine and a motorcycle comprising the four-cycle engine. More particularly, the present invention relates to a construction that enables efficient separation of a blow-by gas from oil.
2. Description of the Related Art
For example, in four-cycle engines mounted in motorcycles, a blow-by gas in some cases flows from a combustion chamber into a crankcase through a gap between a piston and an inner wall of a cylinder block. An internal pressure of the crankcase rises because of the presence of the blow-by gas, and becomes resistance to downward movement of the piston. This undesirably increases a pumping loss and reduces an output in a high-output and high-speed engine. Accordingly, in order to lower the internal pressure of the crankcase, the crankcase is conventionally provided with a gas exhaust outlet to exhaust the blow-by gas to the outside of the crankcase (e.g., Publication of Japanese Examined Patent Application No. Hei. 1-16323).
In general, in four-cycle engines, oil is fed to a journal portion of the crankshaft for lubrication to enable the journal portion and a mounting wall portion mounting the journal portion to smoothly slide relative to each other. Further, the oil is injected to a back surface of the piston to cool the piston. Since the oil fed to these components scatters toward a surrounding region, a gas-oil mixture of the blow-by gas and oil mist is filled in the crankcase. Therefore, if the blow-by gas is exhausted from the interior of the crankcase in order to lower the internal pressure of the crankcase, the oil mist is undesirably exhausted together with the blow-by gas.
In a crankcase that accommodates a transmission along with the crankshaft, a primary gear with a relatively large diameter is mounted on an input shaft end of the transmission, and a gas exhaust outlet is provided at a location (side wall portion of the crankcase) opposite to a side surface of the primary gear. In such a construction, since the primary gear covers a region near the gas exhaust outlet, it is possible to inhibit the oil from mixing into the blow-by gas exhausted from the gas exhaust outlet. Furthermore, in order to remove some oil contained in the blow-by gas exhausted from the gas exhaust outlet, it is known that the gas-oil mixture is guided to a gas-liquid separating chamber to separate the blow-by gas from the oil.
However, in engines having a limited width, such as a four-cycle engine mounted in a motorcycle, it is difficult to provide the gas-liquid separating chamber laterally of the crankcase. So, in many cases, the gas-liquid separating chamber must be located in an extra space distant from the gas exhaust outlet provided on the side wall portion of the crankcase. In these cases, an elongate gas passage is formed in the side wall portion of the crankcase by using a gun drill, or a hole is formed to penetrate the side wall portion of the crankcase to form a gas exhaust outlet, and a gas pipe extends from an outer opening of the gas exhaust outlet of the side wall portion so that the blow-by gas is guided to the gas-liquid separating chamber.